Comparing and Merging Files with GNU diff and patch by David MacKenzie, Paul Eggert, and Richard Stallman Paperback (6"x9"), 120 pages ISBN 0954161750 RRP £12.95 ($19.95) "Well packaged... the quality of information is excellent" --- Linux User and Developer Magazine (Issue 36, Feb 2004) Get a printed copy>>>

Comparing and Merging Files with GNU diff and patch Buy the book here! >>> support free documentation

10.5 Updating Time Stamps on Patched Files

When patch updates a file, it normally sets the file's last-modified time stamp to the current time of day. If you are using patch to track a software distribution, this can cause make to incorrectly conclude that a patched file is out of date. For example, if ‘syntax.c’ depends on ‘syntax.y’, and patch updates ‘syntax.c’ and then ‘syntax.y’, then ‘syntax.c’ will normally appear to be out of date with respect to ‘syntax.y’ even though its contents are actually up to date.

The -Z or --set-utc option causes patch to set a patched file's modification and access times to the time stamps given in context diff headers. If the context diff headers do not specify a time zone, they are assumed to use Coordinated Universal Time (UTC, often known as GMT).

The -T or --set-time option acts like -Z or --set-utc, except that it assumes that the context diff headers' time stamps use local time instead of UTC. This option is not recommended, because patches using local time cannot easily be used by people in other time zones, and because local time stamps are ambiguous when local clocks move backwards during daylight-saving time adjustments. If the context diff headers specify a time zone, this option is equivalent to -Z or --set-utc.

patch normally refrains from setting a file's time stamps if the file's original last-modified time stamp does not match the time given in the diff header, of if the file's contents do not exactly match the patch. However, if the -f or --force option is given, the file's time stamps are set regardless.

Due to the limitations of the current diff format, patch cannot update the times of files whose contents have not changed. Also, if you set file time stamps to values other than the current time of day, you should also remove (e.g., with ‘make clean’) all files that depend on the patched files, so that later invocations of make do not get confused by the patched files' times.